The field of the invention relates to a diesel powered system, such as a train, off highway vehicle, marine and/or stationary diesel powered system and, more particularly, to a circuit for reducing a voltage being developed across a synchronous machine field winding of a diesel-fueled power generation unit.
Diesel powered systems such as, but not limited to, off-road vehicles, marine diesel powered propulsion plants, stationary diesel powered system and rail vehicle systems, or trains, are usually powered by a diesel power unit. With respect to rail vehicle systems, the diesel power unit is part of at least one locomotive and the train further includes a plurality of rail cars, such as freight cars. Locomotives are complex systems with numerous subsystems, with each subsystem being interdependent on other subsystems.
A diesel-electric locomotive typically includes a diesel internal combustion engine coupled to drive a rotor of at least one traction alternator to produce alternating current (AC) electrical power. The traction alternator may be coupled to power one or more electric traction motors. In a typical application, the diesel internal combustion engine is started, or cranked, using the traction alternator. On or more cranking batteries are configured for powering an inverter coupled to the traction alternator to drive the rotor of the alternator, and, in turn, crank the engine until the engine is capable of running on its own. In a typical embodiment, battery current-fed third-harmonic inverters are used for supplying variable frequency alternating current to three-phase stator windings of the traction alternator. In such a system, the rotor of the alternator is coupled to a mechanical load comprising the crankshaft of the engine. Initially, the output torque of the rotor (and hence the magnitude of current in the stator windings) needs to be relatively high in order to start turning the crankshaft. As the rotor accelerates from rest, less torque (and current) is required, and the fundamental frequency of load current increases with speed. In its cranking mode of operation, the inverter supplies the alternator with current of properly varying magnitude and frequency until the engine crankshaft is rotating at a rate that equals or exceeds the minimum speed at which normal running conditions of the engine can be sustained.
FIG. 1 illustrates a schematic block diagram of a prior art electrical power system 10 of a type using traction alternators for cranking an engine 16 of a locomotive. The system 10 includes an alternator field circuit 54 and a third harmonic auxiliary impulse commutated inverter 31 having a pair of dc terminals 19p and 19n connected to a source of relatively smooth direct current. The inverter 31 may include a set of three ac terminals 11, 12, and 13 connected, respectively, to line terminals of three star-connected armature windings on the stator of a rotatable, variable speed, three-phase ac synchronous machine, such as a traction alternator 14. As is suggested by broken lines 23a and 23b, multiple traction motors (not shown) may be connected to receive power from the dc terminals 19p and 19n, if desired. Traction alternator 14 has a rotor 15 that is mechanically coupled to a prime mover, such as a diesel internal combustion engine 16 of a locomotive. The current source for the inverter 31 comprises the combination of a source of voltage, such as a heavy duty electric storage battery 17, in series with an impedance which has appreciable electrical inductance, preferably provided by a dc field winding 18 on rotor 15 of machine 14.
Battery 17 may include a lead-acid or nickel-cadmium type having thirty-two cells and rated 68 volts; and the average magnitude of voltage at its terminals normally does not exceed 76 volts. Its internal resistance is typically in the range of 16 to 37 milliohms. The battery 17 is intended to supply electrical energy for starting the engine 16, and the system shown in FIG. 1 can successfully perform this function with the battery voltage as low as 61 volts.
Synchronous machine 14 may be configured for dual modes of operation, that is, in a powering mode as a generator for supplying alternating current to an electric load circuit that is connected to its stator windings, and in a starting mode as an ac motor for cranking, or starting, engine 16. In its generating mode, rotor 15 of the machine is driven by the crankshaft of engine 16, and field winding 18 is energized by a suitable excitation source 20 (e.g., the rectified output of auxiliary windings on the stator of machine 14) to which it is connected by means of a suitable contactor 47 which is operated by a conventional actuating mechanism 21. For example, the contactor 47 may be opened in the starting mode to disconnect excitation source 20 from the field winding 18 during cranking and closed in the powering mode to excite the field winding 18.
In the motoring mode of operation, rotor 15 of synchronous machine 14 drives the crankshaft of engine 16. Electrical energy is supplied from battery 17 to the windings on both the rotor and the stator of the machine, and rotor 15 generates torque to run the crankshaft and thereby crank the engine 16. As the rotor accelerates from rest, both the frequency and the RMS magnitude of the fundamental alternating voltage waveforms developed at the line terminals of the stator windings (i.e., the back emf) correspondingly increase, while load current (i.e., current in the field and armature windings) decreases in magnitude. Once the rotor is rotating faster than a predetermined rate, which typically is 240 rpm, the engine is presumed to be started and the motoring mode (i.e., engine cranking mode) of operation is discontinued.
The third harmonic auxiliary impulse commutated inverter 31 is operative to convert direct current from battery 17 into variable frequency alternating currents in the three different phases A, B, and C of the three-phase armature windings on the stator of machine 14. The inverter 31 has at least three pairs of alternately conducting main controllable electric valves interconnected and arranged in a three-phase, double-wye bridge configuration between the set of three ac terminals 11, 12, and 13 and the pair of dc terminals 10p and 10n. More particularly, a first pair of valves T1 and T4 are connected in series-aiding fashion from terminal 19p to terminal 19n, and their juncture, comprising terminal 11, is connected to phase A of the stator windings; a second pair of valves T3 and T6 are connected in series-aiding fashion from 19p to 19n, and their juncture, comprising terminal 12, is connected to phase B of the stator windings; and a third pair of valves T5 and T2 are connected in series-aiding fashion from 19p to 19n, and their juncture, comprising terminal 13, is connected to phase C. Each valve preferably comprises at least one solid state unidirectional controlled rectifier popularly known as a thyristor which has a turned on (conducting) state and a turned off (non-conducting) state. In practice, the valves are respectively shunted by conventional snubber circuits (not shown).
The first dc terminal 19p is connected to the relatively positive terminal of battery 17 via a contactor 48, and the second dc terminal 19n is connected to the other terminal of the battery 17 by means of a conductor 25, contactor 51, contactor 52, and a conductor 26. Field winding 18 typically has a resistance in the range of 0.12 to 0.28 ohm and an unsaturated inductance of more than 0.3 henries. Contactor 49, a conductor 27, and resistance 33 comprising two resistors 28 and 29 are connected in parallel with field winding 18 in the load current path between conductors 25 and 26. Resistors 28 and 29 are connected in series, and both have very low ohmic values, for example, the resistance of resistor 28 is approximately 14 milliohms, and the resistance of resistor 29 is shunted by another single-pole contactor 50 which, when closed, reduces the ohmic value of the resistance means 33 to that of the first resistor 28 alone.
An inductance 30 of approximately one milli-henry is connected in series with resistors 28 and 29 between second resistor 29 and conductor 26 in order to smooth the current flowing therethrough. Inductance 30 is shunted by a conventional over voltage protective device 35, the resistance of which is normally very high, but automatically decreases to a negligible amount in substantially instantaneous response to the magnitude of voltage across the inductor rising to a predetermined breakover level (e.g., 750 volts). A similar protective device 32 with bidirectional response is connected across field winding 18. Although protective devices 32, 35 are provided, the excitation source 20 may be disconnected form the field winding 18 so as to prevent damage to the excitation source 20 during cranking from over voltage. A resistor 36 of significant ohmic value (e.g., 100 ohms) is also connected across field winding 18 to enable thyristor “latching” current to bypass the field winding 18 and inductance 30 when battery current starts flowing to pre-charge the inverter's commutation capacitor 45. Capacitor 45 is shunted by a bleeder resistor 46 which effectively keeps the capacitor 45 initially in a substantially discharged state prior to closing contactors 48 and 49 and starting up the illustrated system. Preferably, the commutation capacitor is connected between the juncture M of the auxiliary valves and the neutral S of the three star-connected stator windings.
With field winding 18 in the load current path during engine cranking, synchronous machine 14 will operate with a characteristic similar to that of a series dc motor, namely, high current and hence desirably high starting torque, at low speeds. The resistance means 28 and 29 in parallel with the field winding 18 reduces the ohmic value of resistance that the field winding 18 alone would otherwise introduce in this path, thereby initially allowing a higher magnitude of armature current and later, as speed increases, providing automatic field weakening which permits the machine to run at a higher speed. Initially, load current is limited by the internal resistance of battery 17 as well as other resistance in its path and as speed increases, it is limited by the back emf of the armature (i.e., stator) windings. Thus, load current and torque tend to decrease with increasing speed. A short time after cranking commences, contactor 50 is closed to further reduce the amount of resistance in parallel with the field 18, thereby permitting more load current to flow and hence more torque to be developed at higher speeds as compared to the quantities that would be achieved if the parallel resistance were not so reduced.
When the cranking mode of operation commences, contactor 47 is opened so as to prevent damage to the excitation source 20 during cranking due to voltage spikes, and all of the contactors in the load current path between battery 17 and dc terminals 10p and 10n are closed, except 50. Contactor 50 is closed upon the expiration of a predetermined length of time after cranking commences. Thereafter, in response to the speed of the engine attaining a threshold that marks the conclusion of cranking (e.g., 240 rpm), and therefore the successful starting of the engine 16, all of the previously closed contactors are opened. Upon opening contactors 51, 52, field winding 18 is disconnected from the load current path between conductors 25 and 26, and contactor 47 is then closed by its actuating mechanism 21 in order to reconnect the field to the normal excitation source 20.
Owners and/or operators of locomotives, off-road vehicles, marine diesel powered propulsion plants, and/or stationary diesel powered systems desire to improve reliability and reduce maintenance costs associated with such systems.